Meanwhile, Robb left the remaining Stark-Tully forces in the Riverlands under the command of his uncle Edmure Tully. The war dragged on in the Riverlands in a series of raids, counter-raids, and skirmishes between Edmure's forces based at Riverrun, and Tywin Lannister's forces based at Harrenhal to the east.

Robb's offensive in the Westerlands eventually succeeded in luring a Lannister army led by Gregor Clegane (called "The Mountain That Rides") to march back to the west, and attempt to force a passage over the Red Fork of the Trident River.

Gregor Clegane's forces seized control of Stone Mill, hoping to use it as a crossing site over the Red Fork. Edmure Tully, commanding Robb's forces at and around Riverrun, decided to engage the Mountain's forces and prevent their crossing. The Lannisters withdrew from the area, and Edmure also recaptured Stone Mill.[2]

Tactically, the battle was an amazing success, inflicting two to one losses on the Lannisters and forcing Gregor Clegane to retreat from the Red Fork and back to Harrenhal.[3] Tywin's young nephews, Willem and Martyn Lannister, are taken hostage in the battle.[4]

Strategically, the battle was a significant blunder. Robb Stark's plan was to lure Gregor Clegane's forces further west into the Riverlands where they could be surrounded and destroyed. Edmure's attack instead allowed Clegane and his army to escape. Gregor and his army soon abandoned Harrenhal entirely to take part in the Lannister victory at the Battle of the Blackwater - which would not have happened if Edmure had lured more Lannister forces into the west.[5]

In the A Song of Ice and Fire novels, the battle was fought over an extremely wide front, encompassing fords to the south and north of Riverrun, and featured the entire main Lannister army attempting to cross the river. Edmure's forces were fewer in number but held the strategic high ground on each side of the fords, and were able to subject the Lannister forces to withering arrow fire as they attempted to cross. Ser Jason Mallister commanded the defense of four fords to the south of Riverrun, and Karyl Vance commanded the defense of fords to the north, while Edmure Tully commanded the defense of the fords closest to Riverrun itself. The fiercest assault came at Stone Mill in the center, where the forces commanded by Ser Gregor Clegane himself managed to gain a foothold on the west bank of the river despite suffering terrible losses, but were then driven back when Edmure hit them with his reserves.

The entire campaign was led by Tywin, while Gregor Clegane personally commanded the main thrust at Stone Mill. While the battle was raging, Tywin received word that Stannis Baratheon was about to attack King's Landing. He withdrew from the engagement, turned his army south and marched to the headwaters of the Blackwater Rush. They built barges and skiffs and were able to rapidly head downriver, meeting the army of House Tyrell along the way. This allowed the combined Lannister and Tyrell armies to then attack Stannis's flank at the Battle of the Blackwater, routing him.

Robb later chastises Edmure for his blunder, saying that the Northmen had identified a valley in the western mountains where they planned to lure Tywin Lannister into a trap and destroy his main army, as well as keeping Tywin from aiding King's Landing's defense and thus allowing the Northmen to try and negotiate a peace with Stannis after he took the capital. By preventing his crossing, Edmure won a strategically meaningless short-term victory at the cost of a possible long-term victory for the entire campaign. Edmure angrily pointed out that the blunder was due to Robb not trusting him with his plans.

In the TV series the battle is somewhat smaller in scale and focused specifically around Stone Mill, which is where the main thrust occurred in the novels.

One minor change is about Martyn and Willem Lannister: neither of them was taken captive at the Battle of Stone Mill, and not by the Tullys. Willem and his cousin Tion Frey were captured by the Starks at the Battle of the Whispering Wood, while Martyn was captured at the Battle of Oxcross.